The second-largest data breach in South Korea's history compromised the personal information of more than 13 million subscribers to the Nexon MMO MapleStory, but none of those accounts are held outside of Korea, the publisher said Saturday.

A Nexon spokesman told the Yonhap News agency that the game operates separate servers for each country. The server for South Korea, home of the most MapleStory subscribers, by far, was the only one attacked, Nexon said.

The breach was discovered Thursday. It exposed millions of user IDs, players' names, passwords and their residential registration numbers—the South Korean counterpart to the United States' Social Security number, which many Korean sites require to register an account.

Though passwords and registration numbers were encrypted, users were still advised to change their passwords on other sites. Nexon reported the incident to the Korea Communications Commission and asked for a police investigation.

Yonhap reported that the breach follows an attack in August that exposed personal information of 35 million subscribers to South Korea's Nate search engine and Cyworld social network. The attack also comes as Nexon readies an initial public offering of its Japanese affiliate, a stock sale expected to raise $1.3 billion.

Kotaku contacted Nexon US representatives for a statement. Any comment will be updated here.